This invention relates to the determination of plastic anisotropy in sheet materials and is particularly, though not exlusively, concerned with the rapid determination of the average plastic strain ratio (r), of sheet material, which is an important factor in predicting its `drawability` characteristics.
Hitherto, r has conventionally been determined by cutting three test specimens from a sheet and separately subjecting each to a tensile test, r then being determined from the relationship EQU r=1/4(r.sub.0 +2r.sub.45 +r.sub.90)
where:
the subscripts are the angles between the rolling direction and the axes of the specimens, and
r is the ratio of the natural strains in the width and thickness directions due to tensile elongation.
Several disadvantages are associated with this method however. In particular the specimens must be carefully prepared to be accommodated in the testing machine and care is needed in setting up the machine, and, after straining, ascertaining the length and width strains with the required degree of accuracy. These procedures are time consuming and consequently efforts have recently been made to devise more rapid means for determining this drawability characteristic. Much work has centered on the automation of the tensile testing procedure, for example, by incorporating extensometers for simultaneously monitoring width and thickness strains, but the need for careful specimen preparation in the three different directions remains.